Electrophotographic reproduction devices can generally be divided into copiers or printers. The present invention finds primary utility in printers.
In an electrophotographic printer, a charged photoconductor is selectively discharged by the operation of a print or imaging station, to provide an electrostatic latent image on the photoconductor's surface. This latent image corresponds to the visual image that is to be printed, first by applying toner to the photoconductor, and then by transferring the toner image to the surface of substrate material such as a sheet of plain paper.
Electrophotographic reproduction devices may be constructed to apply toner (i.e. develop) either the photoconductor's discharged area, or to the photoconductor's charged area. The former type device is called a discharged area developing device (i.e., a DAD device), whereas the latter device is called a charged area developing device (i.e., a CAD device). The present invention finds utility in either type of printer.
One of the more popular imaging stations used in electrophotographic printers utilizes one or more scanning laser beams. While the present invention will be described while making reference to a single scanning laser beam, it is to be understood that the spirit and scope of the invention is not to be limited thereto. The invention finds equal utility in printers having a number of scanning laser beams, and with scanning light spot printers of any type.
The broad spirit and scope of the invention are not to be limited to a scanning light beam since, as will be appreciated by those of skill in the art, such a scanning beam generally comprises a moving point, or spot of light, that can be modulated in intensity, for example on and off, to form an electrostatic latent image on the photoconductor.
Thus, the term scanning laser beam, as used herein, is intended to mean any moving point of light to which the photoconductor is sensitive, and which operates to sequentially "print" the small picture elements (PELS) of one or more image rows, as the point of light sequentially scans the photoconductor, image row after image row.
In printers of this type, the image to be printed comprises an electronic image signal that may, for example, reside in the page memory of a data processing system. In this page memory, each photoconductor PEL area that is to be discharged may, again by way of example, be represented by a binary "1", in which case each PEL that is to be left in its charged state would be represented by a binary "0". As the spot o light moves across a photoconductor PEL row, the row content of the page memory signal is gated to control or modulate the spot of light in accurate synchronism with the position of the moving spot of light.
The art has recognized the need to synchronize the gating of the print data signal to laser beam modulator means as a function of the beam's position, both prior to and during each photoconductor scan.
For example, U.S. Pat. No. 3,750,189 discloses the use of a photosensor that is located on an image plane that is established by the physical location of the photoconductor. This photosensor operates to detect the time at which a scanning laser beam begins to scan the image plane. As a result of the detection of the laser beam at this start of scan position, a timing pulse is generated to start the flow of print data to a beam modulator.
By way of another example, U.S. Pat. No. 3,835,249 describes a real time system for synchronizing the flow of print data to a beam modulator. In this device, the scanning beam is first split into two separate beams. A first scanning beam portion is used in conjunction with the modulator to print data onto a photoconductor. A second scanning beam portion is used to scan an optical grating. This second beam portion thereafter impinges upon a detector whose output continuously provides an indication of the first beam's position, as the first beam scans the photoconductor. The output of this detector is used to generate a timing signal that is used to gate print data to the printing beam's modulator. U.S. Pat. Nos. 3,898,627, 4,000,486 and 4,422,099 are generally similar in their teaching.
While prior systems of this general type provide satisfactory operation, the first exemplary system provides only initial beam position information by which print data can thereafter be gated to the laser beam, and the second of these prior systems is expensive in that it requires the use of an optical grating that is an analog of the entire scan length of the laser beam.
Thus, there remains a need in the art for a simple and inexpensive means to provide both position and phase synchronizing for the flow of print data to a scanning spot of light that scans the photoconductor of a reproduction device.